An increased frequency of HLA B27 was confirmed in a series of 118 patients with ankylosing spondylitis. This was significantly higher in patients who acquired the disease at an early age. Other deviating antigen frequencies were found to be due to linkage disequilibrium. An increased frequency of antigen BW16 was noted in B27 negative patients. In ulcerative colitis, a significantly raised incidence of A11 was found, as well as an increased frequency of B18 in Crohn's disease. The only deviating frequency from controls for blood and serum groups was in blood group Kell, which was increased in Crohn's disease.